Paper or paperboard is made by producing an aqueous slurry of cellulosic wood fiber, which may also contain inorganic mineral extenders or pigments, depositing this slurry on a moving papermaking wire or fabric, and forming a sheet from the solid components by draining the water. This process is followed by pressing and drying sections. Organic and inorganic chemicals are often added to the slurry before the sheet forming process to make the papermaking process less costly or more rapid, or to attain specific properties in the final paper product.
The paper industry continuously strives to improve paper quality, increase process speeds, and reduce manufacturing costs. Chemicals are often added to the fibrous slurry before the papermaking wire to improve the drainage and retention performance on the machine wire. These chemicals and chemical programs are called retention and/or drainage aids.
Papermaking retention aids are used to increase the retention of fine furnish solids in the web during the turbulent process of draining and forming the paper web. Without adequate retention of the fine solids, they are either lost to the process effluent or accumulate to excessively high concentrations in the recirculating white water loop and cause production difficulties including deposit buildup and impaired paper machine drainage. Additionally, insufficient retention of the fine solids and the disproportionate quantity of chemical additives which are adsorbed on their surfaces reduces the papermaker's ability to achieve necessary paper quality specifications such as opacity, strength, and sizing.
The dewatering, or drainage, of the fibrous slurry on the papermaking wire is often the limiting step in achieving faster process speed. This is also the stage in the paper papermaking process which determines many paper sheet final properties. Drainage aids will assist in the drainage/dewatering of the pulp slurry.
Typically, a fibrous slurry is deposited on the papermaking wire from the headbox at a consistency (fiber and filler solids content) of 0.5 to 1.5%; the resultant fibrous mat that is removed from the wire at the couch roll and transferred to the pressing section is approximately 20% consistency. Depending upon the machine size and speed, large volumes of water are removed in a short period of time, typically 1 to 3 seconds. The efficient removal of this water is critical in maintaining process speeds.